House Speaker John Boehner (R., Ohio) and Majority Leader Eric Cantor (R., Va.) expressed confidence that the measure will pass.

It is likely to be close.

Conservative groups such as Heritage Action, Club for Growth, and Freedomworks oppose the effort and have urged lawmakers to vote no. Boehner can only afford to lose about 24 Republicans (assuming all Democrats vote no) or “Plan B” will be defeated. A whip count by the Hill put the number of GOP no votes at 25 just hours before the vote.

GOP leaders hope to avoid such a loss, which would resemble the failed July 2011 effort to approve the “Boehner plan” to raise the debt ceiling and apply pressure on Senate Democrats to follow suit.

House leaders appeared just as confident then that they would prevail but came up several votes shy of 218 and were forced to postpone the vote. The House ultimately approved debt ceiling legislation more in line with a proposal offered by Senate Majority Leader Harry Reid (D., Nev.).

Many Republicans in both chambers of Congress felt their failure to win a vote on the Boehner plan destroyed their political leverage with the White House and forced them to accept a worse compromise than they otherwise would have been able to achieve.

For instance, the GOP plan would have only authorized a one-year extension of federal borrowing authority, forcing another debt ceiling showdown prior to the 2012 election. That is something Obama desperately wished to avoid.

House Republicans aides argue now, as they did in 2011, that passing “Plan B” would give them increased leverage in the ongoing debate and pressure Democrats to act to avert, as Boehner put it, “the largest tax hike in American history.”

“It puts [Democrats] in a pretty tough position,” said one GOP leadership aide. “Is Harry Reid really going to refuse to vote on this? Is the president really not going to sign it? Good luck with that.”

Conservative opposition notwithstanding, Boehner’s Plan B has been blessed by influential groups such as the Chamber of Commerce and Americans for Tax Reform, which said in a statement that voting for the measure would not violate its Taxpayer Protection Pledge not to raise taxes.

There is considerable momentum behind the “Plan B” effort, aides say, as more House Republicans have come to accept that it may be the only possible deal on taxes they can achieve with Obama in the White House. Reports of a growing revolt against leadership are similarly unfounded, these aides say.

“This is the best possible outcome,” said the leadership aide. “This president is the most progressive president in our lifetime, and we’re going to get 99.81 percent of what we want on taxes? Great.”

Some sense weakness in the Democratic negotiating position, which could explain the vocal efforts by the White House and Senate Democrats to dismiss Plan B as a nonstarter.

In what is perhaps a foreboding indicator of Plan B’s prospects, 13 House Republicans on Thursday voted against a procedural measure to simply proceed to a vote on the measure, an unusually high number for a procedural vote.